I liked: Fleet subdivisions! Grouping ships into Strike group, Marine lander group, long-range fire support group, Civilian group. Each with separate strategies. Great idea.
I disliked: Having to do it over and over. No saving important creations.
I liked: Detailed Treaties! Fantastic, and loads of fun to think about. In theory.
I disliked: AI drained the joy out of them with clueless responses/proposals. Also couldn't save those treaties for later use.
I liked: Huge tech tree, that does a good job of being better than just a spreadsheet optimizable choice.
I disliked: research interface designed to keep players in the dark. Bug or a feature? Depends on who you ask. I certainly didn't enjoy it.
The stock SE5 tech tree is really terrible. Every weapon increases damage by 5 per level, which destroys any semblance of starting balance between them as tech progresses...
In both SEIV and SEV, it is easier to just use hotkeys than waste time mousing to all the buttons. But yeah, the UI issue of the weird button panel is a big one.